1. Field of the Invention
The present invention relates to a camera apparatus which has a random trigger function, which is connected to a host device through an IEEE 1394 bus.
2. Description of the Related Art
There are camera apparatuses which have a random trigger function of starting exposure by a trigger signal that is supplied from outside at desired timing, as FA (Factory Automation) camera apparatuses which are connected to a host device through an IEEE 1394 bus.
As a technique of maintaining the accuracy of exposure start timing, a camera apparatus having the random trigger function uses a camera control means adopting a hardware trigger method, in which a transmission path for external trigger signals is provided between the host device and the camera apparatus, exposure of which is controlled by the host device, separately from the IEEE 1394 bus, and the camera apparatus receives an external trigger signal on the transmission path and starts exposure (see Patent Literature 1).
However, the camera control means of the hardware trigger method requires a connection interface for the external trigger signal transmission path between the host device and the camera apparatus besides an IEEE 1394 interface, and thus has a problem of complicating a connection interface between the host device and the camera apparatus.
As a camera control means which solves the above problem, there are cameras which have a so-called software trigger function (camera of a software trigger method) which notification of exposure start timing is performed by an IEEE 1394 packet (see Non patent Literature 1).
The camera control means of the software trigger method is realized by providing the host device with a software trigger generation and transmission means, and providing the camera apparatus, which is connected to the host device through an IEEE 1394 bus, with an IEEE 1394 packet reception circuit, a software trigger detection circuit, a synchronization signal generation circuit, an image sensor, an image sensor control circuit, and an IEEE 1394 packet transmission circuit. In the camera apparatus including the above constituent elements, the software trigger detection circuit analyzes an IEEE 1394 packet received by the IEEE 1394 packet reception circuit, and instructs the synchronization signal generation circuit to start exposure when the circuit detects a software trigger packet. The synchronization signal generation circuit notifies the image sensor control circuit of exposure timing in accordance with the instruction. The image sensor control circuit performs exposure control based on the exposure timing for the image sensor. After exposure is finished, the image sensor transmits an obtained image to the image sensor control circuit. The image sensor control circuit converts the obtained image (image data) into an IEEE 1394 packet and transmits the packet to the IEEE 1394 bus through the IEEE 1394 packet transmission circuit.
However, in the camera control means of the software trigger method, the time at which the software trigger packet arrives at the camera apparatus varies (fluctuates) according to the condition of use of the IEEE 1394 bus (traffic condition of the bus band), as illustrated in FIG. 4. This time fluctuation varies the accuracy of the exposure start timing, and thus the software trigger method has a problem of being inferior to the hardware trigger method in imaging performance.
Therefore, in prior art, a camera control, means of the hardware trigger method is used only in an IEEE 1394 camera system which requires exposure start timing of high accuracy, and a camera control means of the software trigger method, hardware connection of which is relatively easy, is used in an IEEE 1394 camera system which does not require exposure start timing of high accuracy.